In an internal combustion engine, the exhaust temperature changes according to the operating state of the engine or the like. When this exhaust temperature is learned, it can be incorporated into varied control and the like. To do this, a temperature sensor for exhaust temperature detection can be simply installed. However, this will lead to increased cost. To cope with this, various techniques to estimate the exhaust temperature have been proposed. For example, the exhaust temperature measuring instrument disclosed in Patent Document 1 is so constructed as to estimate the exhaust temperature as follows: its air to fuel ratio sensor of an electric current limiting type is provided with a heater for activating that sensor; while the heater is being energized, the energization is temporarily interrupted; the exhaust temperature is estimated based on the state of variation in element impedance at that time.
However, the above-mentioned technique disclosed in Patent Document 1 has a problem. In the technique, for example, it is required to forcibly interrupt the energization of a heater. The active state of the air to fuel ratio sensor is changed with the interruption of the heater energization, and the air-fuel ratio cannot be detected with accuracy. Therefore, even though the exhaust temperature can be estimated, the control on the air-fuel ratio of the internal combustion engine and the like are harmfully affected.                Patent Document 1: JP-2000-227364 A        